It is known that in light dispersion or scattering on a matt surface, e.g. on white paper, the intensity of the dispersed light is dependent on the angle of incidence of the light and the observation direction of the dispersed light. It is possible to derive from indicatrix, that with two detectors receiving the dispersed light from a light spot on white paper at different angles, a clear decision can be made as to whether the paper rests flat on the substrate or is strongly inclined (Reflexionsspektroskopie, G. Kortum 1969, Springer-Verlag, Berlin). With suitably chosen angles this phenomenon is similar to the case of a slightly to highly glossy surface. The stray light intensity in the reflection direction is increased strongly, whereas in the remaining directions there is an overall intensity decrease. If the stray light intensity is indicated as a function of the observation angle, this roughly gives the shape of a circle with a lobe in the reflection direction. In other words, when the light strikes the surface obliquely, the stray light intensity in a direction area close to the reflection direction (forward scattering) is greater than the stray light intensity in a direction area close to the direction of the incident light (backward scattering). The difference in the ratio of the light intensities of rearward and forward scattering is increased with a decreasing angle between the surface and the incident light. For certain surface characteristics, there can be a deterioration in the detection compared with white paper, but this is normally small. Only in rare cases are surfaces obtained which diverge from this and, in addition to a forward brightness or gloss in the reflection direction, also have a rearward brightness or gloss in the laser beam direction. A disturbing rearward gloss or brightness does not occur in the case of raw or printed paper.
For example, U.S. Pat. No. 4,450,352 to Olsson discloses a process and an apparatus for measuring this type of reflection with a light source and several, namely at least three detectors (an apparatus with two light sources and two to three stray light detectors is also described in WO-85/05206). Both apparatuses are used as scale flow detectors for the non-contacting counting of printed products and are correspondingly designed. As the light source for illuminating the printed products, exactly parallel laser light is proposed in the first specification and is used for obliquely illuminating the object to be measured and a plurality of photosensitive detectors, preferably three such detectors are used for the comprehensive measurement of the stray light from different directions for faultfree counting of the printed products in a scale flow. As a result considerable costs are involved for ensuring the necessary reliability of the measurement.
According to the prior art, overlapping objects are moved as a scale flow with the edges oriented in the running direction on a plane and the edges of the objects are illuminated in oblique manner with laser light at an acute angle to the plane. The laser light scattered by the objects is measured by at least two measuring cells, which are positioned in such a way that they measure the stray light along at least two propagation directions with different angles to the plane on which the objects are conveyed. One measuring cell is at an acute angle to the plane and the irradiation direction of the laser on the other side of a vertical line through the laser spot, while the other measuring cell is between the propagation direction on the other side of the vertical line and the incident laser beam, so that when an object edge enters said light beam compared with the light reflected back to the first measuring cell, said other measuring cell or cells receive more light. This arrangement illustrates why the necessary measuring reliability cannot be obtained with two detectors through the lack of adequately good resolution and as a result attempts are made to increase this information by using three or more detectors.